Commercial motor vehicles, noncommercial motor vehicles, trailers and semitrailers, motorcycles, and snowmobiles are distinct classes of motor vehicles, and each shall be numbered in separate series. Each series shall carry a distinctive letter selected by the department. Source: SDC 1939, § 44.0105; SL 1943, ch 156 , § 1; SL 1949, ch 162 , § 1; SL 1955, ch 155 ; SL 1977, ch 249 , § 13; SL 1978, ch 224 , § 2; SL 1989, ch 257 , § 98.